The present invention relates to a method for heat treating metal pipes.
It has been well known in the art that when tensile stresses and factors causing corrosion coexist in metal materials such as austenitic stainless steels widely used in nuclear power plants and other chemical plants, corrosion cracks are rapidly accelerated.
In order to improve stresses in such metal pipes, there has been devised and demonstrated a method in which while cooling water is forced to flow in a metal pipe, the metal pipe is subjected to induction heating so that the temperature difference between the outer and inner wall surfaces of the metal pipe causes thermal stresses in excess of a yield point, thereby causing residual compressive stresses in the inner wall surfaces adjacent to the welded joint between metal pipes.
The residual-stress-improvement method of the type described can be applied to metal pipes in simple shape such as straight pipes, but it has been difficult to apply this method to metal pipes in complicated shapes such as double pipes. Even when such method is applied to the residual-stress-improvement treatment of a double pipe, there arise some problems as will be described in detail below.
For instance, assume that a metal pipe 1 which is laid horizontally has a single pipe section 2 and a double pipe section 3 as shown in FIG. 1 and that a cylindrical space 7 ring-shaped in cross section has an enlarged-diameter portion 8 (See FIG. 2). Then even if cooling water is forced into the cylindrical space 7 in the direction as indicated by an arrow in FIG. 1, cooling water cannot completely fill the enlarged-diameter portion or pocket 8 so that a mass of air is left; that is, the so-called air pocket 9 is left in the enlarged-diameter portion 8. As a result, the cooling effect is reduced at the portions adjacent to the air pocket 9 so that the heat treatment conditions in the peripheral direction of the metal pipe 1 vary. As a consequence, there arises the problem that compressive stresses cannot be caused in a stable manner.
The present invention was made to overcome the above and other problems encountered in the conventional methods for heat treating metal pipes and has for its object to eliminate such air pocket as described above in a metal pipe line, thereby minimizing variations in residual stress improvement effects in the peripheral direction and improving reliability of the pipe line.